The present invention relates to electrophotographic copying and image-producing machines and specifically to light-sensitive image transfer drums (hereinafter, "image drums"), used in such machines. More specifically, the invention relates to image drums made of conductive resin and having electrophotographic photoreceptive surfaces.
Image drums made of injection molded conductive resins are known. Also known is the fact that such image drums can be made by adhesively assembling a bearing/drive element of conductive resin to a cylindrical element. Typically, the cylinder element has an open end. A portion of the bearing element is shaped to fit into the cylinder element. Adhesives are used to secure the bearing element to the inside surface of the opening at the end of the cylinder element. The bearing element supports one end of the cylinder element and may incorporate a final drive element to transmit torque from a mechanical drive to the cylinder element to rotate it. The adhesive bond must transmit such torque.
In electrophotographic devices, such as copying machines or laser printers (hereinafter, "main device"), the image drum used to form the copy or print image, is supported in the main device on a cylindrical center shaft, to permit the drum to rotate. The surface of the image drum includes a photosensitive layer on the cylinder element. The material from which the cylinder element is made must conduct any charge accumulated on the surface.
To form an image, the drum is continuously rotated around the cylindrical shaft near a stationary charging element, or elements, to charge the surface. Selected portions of the charged surface are discharged by scanning a light image across the surface. This process creates an electrostatic latent image. The latent image is developed by applying toner to the surface and removing toner that does not adhere to the charged portion of the surface, resulting in a toner image. The toner image is then transferred to some other surface, such as paper, and fixed, usually by heat. During transfer, the image drum makes contact with the surface to which the toner is transferred, generating a frictional load on the drum which must be born by the attachment between the cylinder element and the bearing element. After the transfer of the toner image, toner remaining on the surface of the image drum is cleaned off, often by holding a cleaning element to the surface, again, generating a load that is born by the main device drive.
The image drum is assembled by inserting a portion of the bearing element into the open end of the cylinder element. To allow the cylinder element to be driven by the bearing element, the bearing element may include, for example, a geared flange or other final drive element. The portion of the bearing element fitted into the open end of the cylinder must be affixed to the interior surface of the cylinder to transmit torque applied to the bearing element. In addition, to discharge the portion of the charged surface, the cylinder element must be grounded. It is convenient to ground the cylinder through the bearing element. When the cylinder element is grounded through the bearing element, the attachment between the cylinder and bearing element must permit charge to flow from the cylinder element to the bearing element.
To obtain high quality images, the image drum must rotate smoothly, with uniform speed and without deflection, around the shaft. During the image formation and fixation processes described above, drag on the surface of the image drum is generated by image transfer and cleaning elements which make contact with the surface of the image drum generating a load. This load can be significant and variable, making it important to have a secure drive connection between the bearing element and the cylinder element of the image drum.
With recent reductions in the size and weight of electrophotographic devices, image drums are typically made of organic resin. The bearing element usually consists of a resin flange, that is secured to the resin cylinder with an adhesive. Solvent adhesives are generally used. The cylinder element of the image drum generally consists of a molded body of conductive resin. Compared to conventional drums of metal, such as aluminum alloy, conductive resin drums are lighter in weight and can be manufactured easily by molding. In addition, due to its smooth surface, the molded cylinder element does not require surface polishing as metal elements do. Thus, manufacturing costs of molded resin cylinder elements are lower than for metal cylinder elements. Furthermore, the surfaces of molded cylinder elements are resistant to corrosion and remain stable for long periods of time, even under hot and humid conditions.
Although solvent adhesives are generally used to connect the cylinder element to the flange of the bearing element, the adhesive drying process can consume much time. In addition, adhesive may scatter and stick to the surface of the image drum, or solvent vapors from the adhesive may adversely affect the photosensitive surface of the image drum, particularly its sensitivity. In addition, such adhesives perform poorly in combination with some types of resin used for the flange. Surface preparation may ameliorate adhesion with certain materials, however, such surface preparation may be insufficient. For example, polyacetal resin is currently preferred because it can be molded easily into complex shapes. Since, however, polyacetal resin performs poorly with most types of solvent adhesives, adjacent surfaces of the flange and cylinder are machined to create ridges which increase the adhesive area, thereby improving adhesion. Sufficient fixing strength, however, has still not been attained with this method.
With conductive resins, similar problems occur because conventional fixing methods, employing adhesives, provide inadequate strength. For example, a highly chemical-resistant material such as polyphenylene sulfide (PPS) performs poorly even when fine ridges are machined into the adjacent surfaces of the bearing and cylinder elements to increase the adhesive area.